rsi: Add null check for virtual interfaces in wowlan config

When the "poweroff" command is executed after wowlan enabled, we have
observed a system crash. In the system "poweroff" sequence, network-manager
is sent to inactive state by cleaning up the network interfaces, using
rsi_mac80211_remove_interface() and when driver tries to access those
network interfaces in rsi_wowlan_config() which was invoked by SDIO
shutdown, results in a crash. Added a NULL check before accessing the
network interfaces in rsi_wowlan_config().

Signed-off-by: Sanjay Kumar Konduri <sanjay.konduri@redpinesignals.com>
Signed-off-by: Siva Rebbagondla <siva.rebbagondla@redpinesignals.com>
Signed-off-by: Sushant Kumar Mishra <sushant.mishra@redpinesignals.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>